Hans Kraus

Abstract:

Scintillation properties of PbWO4 and PbMoO4 have been studied as potential cryogenic scintillators for rare event searches. The light output and decay kinetics of PbWO4 and PbMoO4 crystals for excitation with ²⁴¹Am α particles were examined over the temperature range 7300 K. The α/γ ratio was measured with a PbMoO4 crystal scintillator for 5.3 MeV α particles, and the ability to distinguish between signals induced by α particles and γ quanta by pulse shape discrimination was assessed for the PbMoO4 crystal scintillator at 77 K. The energy dependence of the quenching factor for oxygen, molybdenum, tungsten and lead ions at low energy was calculated using a semi-empirical approach with data from the α particle measurements. The study demonstrates the good potential of these scintillators for cryogenic dark matter search and double beta decay experiments. © 2010 Elsevier B.V. All rights reserved.

Abstract:

We studied the feasibility of using sensitization for the improvement of the VUV-to-visible conversion efficiency of red-emitting pentaborate phosphor. It is shown that Tb 3+ ions that exhibit a strong absorption throughout the 160-230 nm wavelength range facilitate enhancement of Eu3 emission in TbM gB 5O 10-Eu at excitation in this spectral range. The maximum increase of the Eu 3+ emission efficiency in a pentaborate host at excitation with 150 nm photons is ca.10%. It is also shown that substitution of Zn for Mg in Gd(Mg,Zn)B 5O 10- Mn phosphor results in strong (up to 2.4 times) enhancement of the intensity of Mn 2+ emission. A possible reason for the observed host sensitization effect is discussed. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

The possibility to use Tb³⁺ as luminescence sensitizer for enhancement of the conversion efficiency of vacuum-ultraviolet (VUV) radiation into visible light was examined. We studied the luminescence properties of K3Tb(PO4)2 and Ba3Tb(PO4)3 activated by Eu³⁺, and of SrAl12O19 co-doped with Mn²⁺ and Tb³⁺ at excitation over the 120 to 300nm wave-length range. It is shown that Tb³⁺ ions, exhibiting a strong absorption band in the VUV, can provide efficient sensitization of Eu³⁺ and Mn²⁺ emissions for excitation in this spectral range, giving rise to intense red and green luminescence, respectively. This study provides a proof for the concept of VUV sensitization, which enables the engineering of luminescence materials with improved efficiency for excitation from a noble gas discharge. © Taylor & Francis Group, LLC.

Abstract:

An increasing number of applications of scintillators at low temperatures, particularly in cryogenic experiments searching for rare events, has motivated the investigation of scintillation properties of materials over a wide temperature range. This paper provides an overview of the latest results on the study of luminescence, absorption and scintillation properties of materials selected for rare event searches so far. These include CaWO4, ZnWO4, CdWO4, MgWO4, CaMoO4, CdMoO4, Bi4Ge3O12, CaF2, MgF2, ZnSe and Al2O3-Ti. We discuss the progress achieved in research and development of these scintillators, both in material preparation and in the understanding of scintillation mechanisms, as well as the underlying physics. To understand the origin of the performance limitation of self-activated scintillators we employed a semi-empirical model of conversion of high energy radiation into light and made appropriate provision for effects of temperature and energy transfer. We conclude that the low-temperature value of the light yield of some modern scintillators, namely CaWO4, CdWO4 and Bi4Ge312, is close to the theoretical limit. Finally, we discuss the advantages and limitations of different materials with emphasis on their application as cryogenic phononscintillation detectors (CPSD) in rare event search experiments. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

Future cryogenic experiments searching for rare events require reliable, efficient and robust techniques for the detection of photons at temperatures well below that to which low-temperature photomultipliers (PMT) were characterised. Motivated by this, we investigated the feasibility of a low-temperature PMT for the detection of scintillation from crystalline scintillators at T=6 K. The scintillation module was composed of a CaWO 4 scintillator and a low-temperature PMT D745B from ET Enterprises. The PMT responsivity was studied at T=290, 77 and 6 K using γ-quanta from ²⁴¹Am (60 keV) and ⁵⁷Co (122 and 136 keV) sources. We have shown that the low-temperature PMT retains its single photon counting ability even at cryogenic temperatures. At T=6 K, the response of the PMT decreases to 51±13% and 27±6%, when assessed in photon counting and pulse height mode, respectively. Due to the light yield increase of the CaWO 4 scintillating crystal, the overall responsivity of the scintillation modules CaWO4PMT is 94±15% (photon counting) and 48±8% (pulse height) when cooling to T=6 K. The dark count rate was found to be 20 s^-1. The energy resolution of the module remains similar to that measured at room temperature using either of the detection modes. It is concluded that commercially available low-temperature PMT are well suited for detection of scintillation light at cryogenic temperatures. © 2010 Elsevier B.V. All rights reserved.